global find and replace uid and gid, HOWTO?

Hi,

I want to change uid and gid for a complete system. (building a system with multiple distros to all use one single homedir with Symlinker)

I try this command:

find /-gid 1003 -exec chgrp 501"{}" \
find: missing argument to `-exec'
but get the above error.

What is wrong with the command syntaxis?

Thanks for your help!!

Erwin